Optical glass

ABSTRACT

An optical glass consisting basically of a B 2  O 3  -- SiO 2  -- La 2  O 3  -- ZrO 2  -- TiO 2  -- ZnO -- BaO system can be produced at a lower melting temperature and in a shorter melting time than it has heretofore been possible by replacing Ta 2  O 5 , Nb 2  O 5  and WO 3  used in the conventional optical glasses with ZnO.

BACKGROUND OF THE INVENTION

This invention relates to an optical glass having refractive indices (Nd) of about 1.71 to about 1.80 and Abbe values (νd) of about 46 to about 38 and consisting basically of a B₂ O₃ -- SiO₂ -- La₂ O₃ -- ZrO₂ -- TiO₂ -- ZnO -- BaO system.

Known in the art of the optical glass having the above described optical properties are ones consisting of a B₂ O₃ -- SiO₂ -- La₂ O₃ -- ZrO₂ -- TiO₂ -- Ta₂ O₅ (and/or Nb₂ O₅ and/or WO₃) -- divalent metal oxides system, as disclosed in U.S. Pat. No. 3,143,432, German Offenlegungsschrift No. 1,496,524 and Japanese Laid-open Publication Nos. 59116/1973, 88106/1973 and 101414/1975. These known optical glasses contain, as their essential ingredients, a B₂ O₃ -- SiO₂ -- La₂ O₃ divalent metal oxides system and further contain ZrO₂, TiO₂ and a relatively large quantity of Ta₂ O₅ etc. which are added for preventing devitrification and achieving desired optical parameters. The optical glasses of this known system, however, are disadvantageous in that they require a relatively high temperature and a long time in melting and, besides, it is difficult to obtain a homogeneous optical glass which is substantially free of seeds.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an optical glass which has eliminated the above described disadvantage of the prior art optical glasses.

Concentrated studies and repeated experiments made by the inventors of the present invention have resulted in a finding that an optical glass of a B₂ O₃ -- SiO₂ -- La₂ O₃ -- ZrO₂ -- TiO₂ -- ZnO -- BaO system in which Ta₂ O₅, Nb₂ O₅ and WO₃ used in the prior art optical glasses have been replaced with ZnO is low in viscosity, substantially free of bubbles or seeds and highly homogeneous and stable without sacrificing the desired optical properties.

According to the present invention, there is provided an optical glass consisting of a composition in weight percent of 8.5 to 20% B₂ O₃, 10 to 19.5% SiO₂, the sum of said B₂ O₃ and SiO₂ being 24 to 33%, 13 to 25% La₂ O₃, 3 to 7.5% ZrO₂, 1 to 8% TiO₂, 5 to 26% ZnO, 25.5 to 37% BaO, to 5% MgO, O to less than 6% CaO, 0 to 10% SrO, the sum of said ZnO, BaO, MgO, CaO and SrO being 36 to 53%, 0 to 5% PbO, 0 to 5% Al₂ O₃ 0 to 2% Li₂ O, 0 to 2% Na₂ O, 0 to 2% K₂ O, the amount of or the sum of the amounts of one or more ingredients of said Li₂ O, Na₂ O and K₂ O being 0 to 2%, and 0 to 1% of at least one oxide of the group consisting of As₂ O₃ and Sb₂ O₃.

DETAILED DESCRIPTION OF THE INVENTION

In the optical glass made according to the present invention, the above mentioned composition is selected for reasons stated below.

As is well known, B₂ O₃ and SiO₂ are essential glass forming ingredients. If the content of B₂ O₃ is less than 8.5%, a devitrification tendency increases whereas if the content of B₂ O₃ exceeds 20%, the refractive index decreases resulting in failure to achieve the desired optical properties. If the content of SiO₂ is less than 10%, the devitrification tendency increases whereas if the content of SiO₂ exceeds 19.5%, the refractive index decreases resulting in failure to achieve the required optical properties. If the sum of B₂ O₃ and SiO₂ is less than 24%, forming of glass becomes difficult whereas if the sum of B₂ O₃ and SiO₂ exceeds 33%, the desired optical properties cannot be realized.

La₂ O₃ contributes to increasing the refractive index and decreasing the devitrification tendency and if the content of La₂ O₃ is less than 13%, the desired optical properties cannot be achieved whereas increasing the content of La₂ O₃ above 25% increases the devitrification tendency and results in failure to obtain a stable optical glass.

ZrO₂ contributes to increasing the refractive index and decreasing the devitrification tendency. If the content of ZrO₂ is less than 3%, the devitrification tendency increases whereas if the content of ZrO₂ exceeds 7.5%, the raw material becomes harder to melt resulting in occurrence of inhomogeneity in the produced glass.

TiO₂ is a useful ingredient for preventing devitrification, improving chemical durability and maintaining the desired optical properties. If the content of TiO₂ is less than 1%, the above mentioned effects of TiO₂ are extremely reduced whereas if the content of TiO₂ exceeds 8%, the produced glass is colored to an undesirable degree.

ZnO contributes to decreasing viscosity of glass in melting and thereby facilitating elimination of bubbles or seeds from the glass, ZnO further contributes to facilitating melting of SiO₂ into glass with a resulting prevention of devitrification and to improving chemical durability of the glass. If the content of ZnO is less than 5%, the above described effects of ZnO are extremely reduced whereas if the content of ZnO exceeds 26%, the devitrification tendency increases.

BaO is a useful ingredient for increasing the refractive index. If the content of BaO is less than 25.5%, the refractive index required in the present invention cannot be achieved whereas increasing the content of BaO above 37% increases the devitrification tendency and a stable optical glass cannot be obtained.

MgO, CaO and SrO have a function to reduce viscosity of glass in melting and also have a function to facilitate melting of the SiO₂ raw material. Further, MgO is useful for decreasing the devitrification tendency and increasing a water-resistance characteristic while CaO and SrO are useful for decreasing dispersion of the refractive index. For making a stable glass with sufficiently small devitrification tendency, the content of MgO must be up to 5%, that of CaO less than 6% and that of SrO up to 10%.

If the sum of ZnO, BaO, MgO, CaO and SrO is less than 36%, the optical properties required by the optical glass according to the present invention cannot be realized whereas if this sum exceeds 53%, the devitrification tendency increases and chemical durability decreases.

PbO is useful for increasing the refractive index and facilitating melting of SiO₂ but the glass will be undesirably colored if the content of PbO exceeds 5%.

Al₂ O₃ is useful for improving chemical durability and facilitating melting of the SiO₂ material but increasing Al₂ O₃ above 5% increases the devitrification tendency.

Li₂ O, Na₂ O and K₂ O also are useful for facilitating melting of SiO₂ but if the content of one of these oxides or the sum of two or more of these oxides exceeds 2%, chemical durability decreases and the devitrification tendency increases.

As₂ O₃ and/or Sb₂ O₃ has a function of facilitating elimination of seeds in melting of glass but the content thereof should be up to 1%.

Compositions of the optical glass made according to the present invention are shown by way of example in Table 1 and compositions of the prior art optical glass having correspondingly substantially equal optical constants (Nd, θd) and containing Ta₂ O₅, Nb₂ O₅ and WO₃ etc. are shown in Table 2. In the respective tables, each component is shown in weight percent. These tables show numbers of seeds contained in the glasses 1.5 and 2.5 hours respectively after the component materials are poured into a platinum crucible which is maintained at a constant temperature of 1300° C. Each of the numbers of seeds in these tables is shown in terms of an average number of seeds in each unit section. This unit section is obtained by dividing a glass specimen into pieces each having a volume of 50cc, which glass specimen is obtained by mixing 1 kg of glass component materials, melting the mixture, pouring the melt in a mould and thereafter annealing the melt.

It will be understood from Table 1 and Table 2 that the glass compositions according to the present invention generally have a much smaller number of seeds remaining after melting than the prior art optical glass compositions and therefore can be readily refined. It has also been found that the glass compositions according to the present invention are lower in viscosity (log η) at 1300° C than the prior art compositions by 0.5 to 0.7.

Some other examples of the composition of the optical glass according to the present invention are shown in Table 3 in the same manner as in Table 1 except for the number of seeds which is a number measured 2.5 hour after filling in of the component materials into the crucible only. It will be appreciated that the optical glass shown in Table 3 can be refined as easily as the optical glass shown in Table 1.

                                      Table 1                                      __________________________________________________________________________                                                         Number of                                                                      seeds                                                                Optical   1.5                                                                                 2.5 hr                (Weight percent)                          properties                                                                               after                                                                               after                 No.                                                                               B.sub.2 O.sub.3                                                                    SiO.sub.2                                                                          La.sub.2 O.sub.3                                                                    ZrO.sub.2                                                                          TiO.sub.2                                                                          ZnO BaO Al.sub.2 O.sub.3                                                                    Li.sub.2 O.sub.3                                                                    Nd   νd                                                                               melting                                                                             melting               __________________________________________________________________________     1  15.0 14.0                                                                              22.0 6.0 5.0 7.0 31.0          1.7619                                                                              42.7 14   2                     2  15.0 17.0                                                                              14.0 5.0 7.0 14.0                                                                               26.0                                                                               2.0       1.7328                                                                              38.3 57   3                     3  14.0 11.0                                                                              24.0 7.0 2.5 15.0                                                                               26.5          1.7553                                                                              45.6  8   0                     4  13.0 12.5                                                                              24.5 6.0 8.0 8.0 28.0          1.7999                                                                              38.6 10   0                     5  8.5  18.5                                                                              13.5 6.0 1.0 26.0                                                                               26.0     0.5  1.7318                                                                              45.0 16   2                     6  14.0 17.0                                                                              18.5 3.0 7.5 10.0                                                                               30.0          1.7604                                                                              39.2 31   2                     7  9.5  19.5                                                                              15.0 5.0 2.0 19.0                                                                               25.0                                                                               5.0       1.7118                                                                              43.0 38   4                     __________________________________________________________________________

                                      Table 2                                      __________________________________________________________________________                                                  Number of                                                                      seeds                                                                   Optical                                                                               1.5 hr 2.5 hr                     Prior arts (Weight percent)           properties                                                                            after  after                      No.                                                                               B.sub.2 O.sub.3                                                                   SiO.sub.2                                                                         La.sub.2 O.sub.3                                                                   ZrO.sub.2                                                                          TiO.sub.2                                                                         Ta.sub.2 O.sub.5                                                                   Nb.sub.2 O.sub.5                                                                   WO.sub.3                                                                          BaO    Nd  νd                                                                             melting                                                                               melting                    __________________________________________________________________________     1' 14.5                                                                              17.5                                                                              24.5                                                                               6.0 5.5                                                                               3.0        26.0                                                                              CaO 1.7620                                                                             42.6                                                                              innumerable                                                                           innumerable                                                  3.0                                          2' 17.0                                                                              16.0                                                                              17.0                                                                               7.0 5.0                                                                               9.0     2.0                                                                               24.0                                                                              Al.sub.2 O.sub.3                                                                   1.7328                                                                             38.3                                                                              "      73                                                           3.0                                          3' 25.5                                                                              10.0                                                                              39.5                                                                               8.0    10.0    4.0   CaO 1.7551                                                                             45.6                                                                              "      90                                                           3.0                                          4' 14.0                                                                              13.0                                                                              24.7                                                                               8.0 8.3                                                                               3.0        27.0                                                                              CaO 1.8014                                                                             38.6                                                                              "      54                                                           2.0                                          5' 27.0                                                                               6.5                                                                              25.5                                                                               2.0    8.0 2.5 2.5                                                                               25.0                                                                              Al.sub.2 O.sub.3                                                                   1.7316                                                                             45.1                                                                              "      82                                                           1.0                                          6' 15.0                                                                              16.0                                                                              21.0                                                                               3.5 6.0                                                                               5.5     3.0                                                                               25.0                                                                              MgO 1.7609                                                                             39.2                                                                              "      innumerable                                                  5.0                                          7' 18.5                                                                              19.0                                                                              24.5                                                                               5.0 2.0                                                                               3.0 2.0 1.0                                                                               20.0                                                                              SrO 1.7123                                                                             42.9                                                                              "      "                                                            5.0                                          __________________________________________________________________________

                                      Table 3                                      __________________________________________________________________________                                                 Number of                                                               Optical                                                                               seeds                              (weight percent)                     properties                                                                            2.5hr after                        No.                                                                               B.sub.2 O.sub.3                                                                   SiO.sub.2                                                                         La.sub.2 O.sub.3                                                                   ZrO.sub.2                                                                          TiO.sub.2                                                                         ZnO                                                                               BaO           Nd  νD                                                                             melting                            __________________________________________________________________________      1 10.0                                                                              17.5                                                                              20.5                                                                               6.0 3.5                                                                                8.0                                                                              28.0                                                                              CaO                                                                               Al.sub.2 O.sub.3                                                                       1.7442                                                                             44.0                                                                              4                                                            5.5                                                                               1.0                                                2 20.0                                                                              10.0                                                                              17.0                                                                               7.0 3.5                                                                               10.0                                                                              26.5                                                                              SrO        1.7399                                                                             44.6                                                                              1                                                            6.0                                                   3 11.5                                                                              15.5                                                                              13.0                                                                               6.0 3.0                                                                               18.0                                                                              27.5                                                                              MgO    As.sub.2 O.sub.3                                                                   1.7381                                                                             43.2                                                                              3                                                            5.0    0.5                                            4 10.5                                                                              17.0                                                                              25.0                                                                               3.5 3.5                                                                                5.0                                                                              32.5                                                                              CaO                                                                               Al.sub.2 O.sub.3                                                                       1.7460                                                                             44.8                                                                              6                                                            2.5                                                                               0.5                                                5 10.5                                                                              14.0                                                                              17.0                                                                               4.5 3.0                                                                               15.0                                                                              36.0          1.7490                                                                             43.4                                                                              2                                   6 12.0                                                                              17.5                                                                              18.9                                                                               4.0 3.0                                                                                8.0                                                                              31.5  PbO As.sub.2 O.sub.3                                                                   1.7605                                                                             41.2                                                                              4                                                               5.0 0.1                                            7 16.5                                                                              10.5                                                                              14.0                                                                               4.0 4.0                                                                               21.0                                                                              28.0      K.sub.2 O                                                                          1.7331                                                                             43.0                                                                              0                                                                   2.0                                            8 10.5                                                                              18.0                                                                              17.0                                                                               4.5 5.0                                                                               13.5                                                                              30.0  Na.sub.2 O                                                                             1.7455                                                                             41.6                                                                              5                                                               1.5                                                9 14.0                                                                              13.5                                                                              23.0                                                                               6.0 6.0                                                                                7.5                                                                              30.0          1.7767                                                                             40.7                                                                              0                                  10 12.5                                                                              15.0                                                                              19.5                                                                               6.0 1.0                                                                               20.0                                                                              26.0          1.7378                                                                             45.9                                                                              1                                  11 10.0                                                                              14.0                                                                              18.0                                                                               5.0 4.0                                                                               14.0                                                                              35.0          1.7590                                                                             42.0                                                                              1                                  12 18.0                                                                              15.0                                                                              17.0                                                                               6.0 4.5                                                                               12.0                                                                              26.0                                                                              Li.sub.2 O                                                                        Na.sub.2 O                                                                         K.sub.2 O                                                                          1.7275                                                                             43.0                                                                              0                                                            0.5                                                                               0.5 0.5                                           13 11.5                                                                              14.0                                                                              15.0                                                                               4.0 2.5                                                                               16.0                                                                              37.0          1.7385                                                                             43.7                                                                              2                                  14 14.5                                                                              16.5                                                                              20.5                                                                               7.5 3.5                                                                               12.0                                                                              25.5          1.7421                                                                             44.3                                                                              3                                  15 12.0                                                                              13.5                                                                              17.0                                                                               4.5 3.0                                                                               14.0                                                                              26.0                                                                              SrO        1.7315                                                                             43.6                                                                              1                                                            10.0                                                 16 14.5                                                                              13.5                                                                              16.0                                                                               5.0 4.0                                                                               18.0                                                                              28.0      Sb.sub.2 O.sub.3                                                                   1.7448                                                                             40.8                                                                              1                                                                   1.0                                           __________________________________________________________________________

As described in the foregoing, the optical glass according to the present invention has optical properties of refractive indices (Nd) of about 1.71 to about 1.80 and Abbe values (νd) of about 46 to about 38 and consists basically of a B₂ O₃ -- SiO₂ -- La₂ O₃ -- ZrO₂ -- TiO₂ -- ZnO -- BaO system without using Ta₂ O₅, Nb₂ O₅, WO₃ and other oxides contained in the prior art optical glasses having substantially equal optical properties. The optical glass according to the invention is lower in viscosity than the prior art optical glasses and has achieved a drop in the melting temperature from about 1350° - 1400° C to about 1250° - 1350° C. The optical glass according to the invention has also achieved sufficient homogenization and elimination of seeds with curtailment of a melting time including time required for stirring by about 30% to 50%. Accordingly, wear and tear of a glass melting apparatus can be reduced and yield of glass increased. Furthermore, heat consumption and working time can be saved and costs of raw materials can be greatly reduced. 

What is claimed is:
 1. An optical glass consisting of a composition in weight percent of 8.5 to 20% B₂ O₃, 10 to 19.5% SiO₂, the sum of said B₂ O₃ and SiO₂ being 24 to 33%, 13 to 25% La₂ O₃, 3 to 7.5% ZrO₂, 1 to 8% TiO₂, 5 to 26% ZnO, 25.5 to 37% BaO, 0 to 5% MgO, 0 to less than 6% CaO, 0 to 10% SrO, the sum of said ZnO, BaO, MgO, CaO and SrO being 36 to 53%, 0 to 5% PbO, 0 to 5% Al₂ O₃, 0 to 2% Li₂ O, 0 to 2% Na₂ O, 0 to 2% K₂ O, the amount of or the sum of the amounts of one or more ingredients of said Li₂ O, Na₂ O and K₂ O being 0 to 2%, and 0 to 1% of at least one oxide of the group consisting of As₂ O₃ and Sb₂ O₃. 